Project Summary Sleep disordered breathing occurs more commonly in the pregnant population; however, there has been no research to date that has assessed specific predictors of pregnancy-onset development of this disorder. It is possible that, with the exposure to pregnancy physiology, women with a specific phenotype are more likely to develop obstructive sleep apnea. The aim of this project is to 1) assess anthropometric and body composition variables in women with and without obstructive sleep apnea in early pregnancy; 2) examine physiologic and biological measures longitudinally and relate changes to the development of new-onset obstructive sleep apnea in pregnancy; and 3) evaluate the role of rostral fluid shifts around sleep in the pathogenesis of obstructive sleep apnea. The study will enroll pregnant women with obesity. Participants will have measurements of body composition, relevant anthropometric measures, biological markers ?such as female hormones, adipokines and inflammatory markers-, pulmonary function tests and upper airway patency measurements, followed by in-home sleep apnea testing. Women with obstructive sleep apnea will be compared to those without. Women without sleep apnea will then be followed longitudinally for the development of obstructive sleep apnea during pregnancy by repeat home sleep apnea testing. Early pregnancy measures and changes in these measures during gestation will be assessed as predictors of new onset obstructive sleep apnea and related to apnea hypopnea indices and other polysomnographic measures of obstructive sleep apnea obtained on repeat in-home sleep studies. This aim will facilitate the identification of specific pregnancy dynamic changes that predispose some women at risk, but not others, to develop obstructive sleep apnea in pregnancy. To further understand the role of fluid shifts during sleep, a subgroup of women will be invited to have an in-laboratory polysomnography test where measurement of leg and neck circumference will be obtained in addition to an assessment of fluid shifts from the lower extremities. The knowledge gained from this study will help us identify factors that would place a woman at risk for developing de novo obstructive sleep apnea during pregnancy paving the way for targeted screening strategies. Such research will also facilitate the identification of women at risk for these complications to be pursued in interventional trials.